Needing the pH level to be appropriate prior to shocking

There are two issues. First, the PH test doesn't work reliably when FC is above 10. Since shocking generally keeps FC above 10, you won't be able to adjust the PH during shocking. Most pools have some PH drift, so you want to make sure PH is appropriate before you start as you will not be able to adjust the PH for several days (or occasionally weeks). Second, extremely low or extremely high PH can prevent chlorine from being effective. If your PH is too far out of range and you don't correct that shocking won't work.
 
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